HDFS-7073. Allow falling back to a non-SASL connection on DataTransferProtocol in several edge cases. Contributed by Chris Nauroth.

(cherry picked from commit f85cc14eb49a46e81d2edcdc1ffe4d0852f193a5)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSConfigKeys.java
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/balancer/Dispatcher.java
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/balancer/NameNodeConnector.java